The present invention relates to the measurement of contaminant particles deposited on a substrate and/or the measurement of deposition/etching film thickness on a substrate, such as a semiconductor wafer or the like, in a device fabrication process system during chemical vapor deposition (CVD), physical vapor deposition (PVD) or an etching process.
To control the quality of manufacturing, semiconductor wafers or the like are inspected for particles and defects by scanning the entire wafer surface with apparatus similar to that described in U.S. Pat. Nos. 4,378,159 to L. Galbraith, 5,076,692 to A. Neukermans et al., 5,083,035 to J. Pecen et al., 5,189,481 to P. Jann et al. and 5,416,594 to K. Gross et al. These methods require the wafer to be transferred from a process system, such as a PVD chamber, a CVD chamber or an etching chamber to a wafer scanner for inspection.
A process system for PVD, CVD or etching usually operates at a negative pressure (vacuum) ranging from one tenth of a millitorr to a few torr, while a wafer scanner typically operates at atmospheric pressure. Transporting wafers from a vacuum to atmospheric pressure and from room atmospheric pressure to a vacuum requires the use of a so called "load-lock" chamber device. The load-lock is an intermediate chamber which has two doors or gate valves, one connected to the vacuum process system and the other to ambient or room conditions. When a wafer or a set of wafers is loaded, the door of the load-lock chamber leading to the vacuum process system is closed, and the loading door to ambient is opened for wafer loading at ambient pressure. After loading, the loading door is closed and the load-lock chamber is pumped down to a vacuum level desired. When the pressure in the load-lock chamber is reduced to a level comparable to that in the process system, the exit door of the load-lock chamber to the process system is opened to allow the wafers to be transferred to the process system.
The pump down speed in the load-lock chamber is usually limited, because a high pump down rate may result in rapid changes in temperature as well as pressure. Rapid temperature change will cause vapor condensation to form contaminant particles. A typical pump down process may take up to a few minutes to complete. For comparison, the deposition time is much shorter. Typical metallic films and dielectric films may have a film thickness ranging from one hundred to a few thousand angstroms and typical deposition rates of a PVD or CVD process range from a few hundred to a few thousand angstroms per minute. The deposition time for such a film thickness in a typical PVD or CVD process is, therefore, on the order of under one minute or so.
Although a slow pump down speed can avoid vapor condensation and particle nucleation in load-locks, the speed is often set at or close to the highest tolerable limit in order to reduce the pump down time. The high pump down speed desired for efficiency increases the risk of forming contaminant particles in the load-lock, thus increasing the risk of wafers being contaminated during their transfer through a load-lock.
Advanced processing systems often have multiple deposition or etching chambers operated under the same, or similar vacuum conditions. One example of prior art is the vacuum processing apparatus shown in FIG. 1 of this application and described in U.S. Pat. No. 4,962,063 to D. Maydan et al. The vacuum processing apparatus comprises a number of deposition and etching chambers 102-108. Each of the chambers 102-108 may be used to carry out chemical vapor deposition, plasma etching, and other deposition and etching processes on a wafer normally under vacuum. Chambers 102-108 are each connected, via interlocks or slit valves 110 to a central chamber 116. The central chamber 116 houses a robot mechanism 120 for transporting the wafers within the vacuum apparatus from and to various processing chamber 102-108 and an internal wafer storage 150 (the load-lock) without breaking vacuum in the system. A supply of wafers outside the chamber is shown at 160. When a wafer completes one process in one chamber of the processing system, it can be internally transferred to another chamber for another process without leaving the vacuum system. This eliminates the need to transport the wafer through the load-lock to the other chamber. The production process is, therefore, simplified and production yield is increased. However, if the wafer needs to be inspected for particle contamination, film thickness, etching thickness or defects after each process, the wafer still has to be transferred through the load-lock to an inspection system located outside the vacuum system and operated at atmospheric (room) pressure for inspection. Such a transfer requires significant time to accomplish and reduces the advantage gained by using a multi-chamber process apparatus.
The advantages of a multi-chamber process apparatus can be fully realized if wafers can be inspected after one process while inside the processing system under the same, or similar vacuum conditions.
A processing system, such as a PVD, a CVD or an etch chamber is routinely shut down for maintenance or for a process recipe change. When the system is restarted, the conventional method of adjusting the processing system is to use so called monitor wafers. A deposition or etching process is first performed on a monitor wafer in the processing chamber. The wafer is then transferred to a wafer inspection apparatus to measure deposited particles, defects, deposition film thickness, etching thickness or the like. The information from the wafer inspection is then used to adjust the processing parameters. A typical start up for a single chamber requires processing a few to ten monitoring wafers, depending on the operating conditions and the operator's skill. A typical start up for a five-chamber vacuum process apparatus may need to process up to 50 monitor wafers, as each of the process chamber has to be individually adjusted. The start up is also a time consuming process since each of the monitor wafers has to be transferred to an outside machine for inspection after deposition or etching. The cost of using the monitor wafers is not prohibitive for small diameter wafers, which usually cost a few dollars per wafer. However, as the wafer diameter increases the cost associated with the monitor wafers significantly increases. Currently an 8 inch wafer costs approximately $30 and a 12 inch wafer costs $500 to $1,000. Reduction in monitor wafer usage is thus desired in the semiconductor device fabrication industry.
A monitor wafer is usually inspected before being loaded into the processing chamber. If an inspected monitor wafer is contaminated in a load-lock and then further contaminated in a fabrication process, a wafer inspection tool cannot distinguish how many contaminant particles are added to the wafer in the load-lock and how many are added during the fabrication step. One method to solve the problem is to leave a monitor wafer in the load lock during the process and inspect the wafer for reference. The start up time and cost for a process tool is further increased due to inaccurate information about the process contamination and the increased requirement for wafers being transferred back and forth through the load-lock between the process system and the wafer inspection tool.
The time required for a process system start up can be significantly reduced if a monitor wafer can be inspected inside the processing system. The reduction in process time is achieved by reducing the number of wafer transfers through the load-lock, which requires a pump down once for each wafer cassette transferred. The usage of monitor wafers is also reduced as the contamination risk in the load-lock is eliminated. A monitor wafer can be repeatedly used until the wafer is over loaded with particles or films. For example, assume that a monitor wafer is initially clean and undergoes a deposition process. After the deposition process if the wafer is inspected in an inspection chamber within the vacuum system and 50 particles are found on the wafer surface, it is known the 50 particles are contaminant particles deposited on the wafer during the deposition process. The processing system is then adjusted accordingly. After adjusting, the wafer is sent back to the processing chamber for another processing. When the process is finished, the monitor wafer is again sent to the inspection chamber for inspection. This time, there are a total of 75 particles found on the monitor wafer. The net addition in the second process is then 25 particles. The processing system will be adjusted again and the same monitor wafer can be used for additional tests.
In addition to scanning a wafer surface for contamination control, some methods have been developed for in-situ real time measurement of particles in process systems. U.S. Pat. No. 5,271,264 to S. Chanayem describes a method and apparatus of detecting particles in a process system exhaust. An in-situ particle monitor is placed down stream of a turbo pump for the vacuum system. However, measuring particles in an exhaust line of a process chamber gives little useful information on particles inside the vacuum process chamber. According to some industry experts, the correlation is poor between the measured particle concentration in an exhaust line and the particles deposited on a wafer after a process. U.S. Pat. No. 5,347,138 to D. Aqui describes a method using a non-invasive particle monitor to detect particles in a process chamber. In this method, a laser beam of an oval cross section is directed through a transparent window from a source outside a down sputter process chamber into the process chamber for detecting particles suspended in the plasma region during the process. Long and narrow shield tubes, each having a length of no less than three mean free paths of the gas molecules in the process chamber and a width less than one mean free path of the gas molecules in the process chamber are used to prevent metal deposition on the surface of the transparent window. Without the shield tube, the transparent window used for passing the laser beam will be soon contaminated by the metal deposition during the sputter process. However, with the shield tube, the measurement can only be made through the tube opening, whose cross sectional area is only a few square millimeters. The measurement results, therefore, do not represent the actual situation in the process chamber. The probability of generating false signal from such a measurement is high. In a semiconductor process, it is cost prohibitive to tolerate a false signal from the process tool because a false signal often results in the shut down of a process line. As described previously, re-start of a process tool is time consuming and very costly.
In some wafer processing, thin metallic and/or dielectric films are deposited on a wafer surface. The typical film deposition thickness ranges from 50 angstroms to several thousand angstroms. Knowledge of film thickness and uniformity of the thickness are desired so that the deposited layers will have the desired properties. Currently, the commonly used instrument for measuring film thickness and the uniformity of the film thickness is the ellipsometer. Most ellipsometers can not operate inside the vacuum processing apparatus for various reasons. Some in-situ measurement of film thickness methods have also been developed. U.S. Pat. No. 5,220,405 to S. Barbee describes an interferometer for measuring thin film thickness changes. U.S. Pat. No. 5,258,824 to D. Carlson et al. describes a method and apparatus used to determine the thickness of a layer deposition on a specimen. The intensity of radiation emitted by the wafer from its surface and the temperature of the silicon wafer are measured in the '824 patent. The variation in the intensity of radiation emission due to variation of the temperature is subtracted from the intensity of radiation emitted. The resultant signal is then used to calculate the thickness of the thin film.
U.S. Pat. No. 5,313,044 to H. Massoud describes a method using an ellipsometer to measure the change in polarization of light upon reflection from a wafer sample to determine the thickness of a thin film.
U.S. Pat. No. 5,450,205 to H. Sawin describes a method to measure the absolute thickness of a thin film using a CCD camera. The present invention relates to methods and apparatus to inspect wafers for particles and thin film parameters inside the vacuum system to eliminate or reduce the need for wafer transfer through a vacuum load-lock to an outside instrument for inspection.